Hot melt adhesives are thermoplastic materials that can be heated to a melt and then applied to various substrates. A bond is formed upon cooling and resolidification. Among the most widely used thermoplastic polymers in hot melt adhesives is ethylene-vinyl acetate copolymer (“EVA”) which is combined with a variety of plasticizers, tackifiers, antioxidants, waxes, and extenders for purposes of improving and/or controlling the viscosity, adhesive properties, shelf-life, stability and cost. Plasticizers have typically included such compounds as polybutenes and phthalates, tackifiers have typically included such compositions as rosin esters and hydrocarbon resins, antioxidants are frequently based upon the known hindered phenol compounds, and wax helps to reduce the melt viscosity in addition to reducing costs.
These hot melt adhesives have the drawback of often becoming brittle below the glass-transition temperature. Historically, ethylene based semi-crystalline polymers like polyethylene and ethylene vinyl acetate copolymer (EVA) have been used in various adhesive applications; however, such polymers have many problems in their end use applications. For example, semi-crystalline linear low density polyethylene (LLDPE) can be used in hot melt adhesive applications where the crystalline network formed on cooling makes a good adhesive free of tack, but the high level of crystallinity causes the material to be brittle. For this reason other monomers, such as vinyl acetate (VA), or alpha-olefins are often co-polymerized with ethylene to break up some of the crystallinity and soften the adhesive. Thus the use of hot melt adhesives based upon EVA is limited when low temperature conditions of use are desired.
Styrene block copolymers (“SBC”) are independently known as an important class of base polymers for adhesive compositions, particularly for such uses as in hot melt pressure sensitive adhesives in tapes, label stock, diaper assembly and the like. However, because of higher melt viscosities than EVA based compositions, SBC based adhesive compositions are not typically used for packaging where high-speed application is economically desirable.
Certain adhesive composition blends of SBC and EVA are known, even though the base polymers are largely incompatible, in the sense of not being able to form stable blends largely free of separation or stratification and resulting nonuniformity of properties. U.S. Pat. No. 4,345,349 describes book-binding hot melt adhesive compositions prepared from 15-30 wt % SBC, 5-10 wt % EVA, 25-40 wt % rosin ester tackifier, 25-35 wt % wax diluent and 0.5-3 wt % of a stabilizer, e.g., hindered phenol compound. The weight ratio of SBC to the ethylene vinyl acetate copolymer is from 1.75/1 to 6/1. The low-temperature flexibility improves by increasing the amount of SBC in the composition and using a high softening point tackifier or high melting point wax shortens setting speed. Setting time, in order to be useful in the described bookbinding process, is to be within 30 seconds, and times within 26 seconds are exemplified. U.S. Pat. No. 4,394,915 describes a hot melt adhesive particularly suitable for poly(ethylene-terephthalate) (PET) bottle assemblies comprising typically 20-40 wt % SBC, 5-20 wt % EVA, 30-60 wt % tackifying resin, 10-30 wt % wax or oil, and 0.1-4 wt % stabilizer. The tackifying resin can be any of a number of available rosins or resins, including the aliphatic petroleum resins, but is preferably a polymerized tall oil rosin.
PCT/US97/04161 teaches the use of ethylene based copolymers as hot melt adhesive and these materials are useful in some applications, but suffer in that they have higher melt viscosity, poorer processing and poorer adhesion to some types of surfaces than propylene based copolymers. U.S. Pat. No. 5,118,762 addresses the industrial need for hot melt adhesives that have a low melt viscosity and high temperature resistance to shear. The solution in this document is the use of a predominantly branched styrene-isoprene-styrene (SIS) triblock copolymer with a tackifying resin that is compatible with the elastomeric isoprene block, e.g., diene-olefin copolymer resins, rosin esters or saturated petroleum resins, e.g., hydrogenated dicyclopentadiene resins such as ESCOREZ® 5000 series resins of the ExxonMobil Chemical Company.
The present invention is directed in general to providing improved adhesive compositions, and processes or methods for making such compositions.